Since Rockstar North are currently working on Agent, it's unlikely we'll hear much about a full new GTA game until at least after Agent has been released. Apparently Agent is due out this year but we know almost nothing about it and it's nearly September, I wouldn't be too surprised if it got pushed back as screen shots and info usually starts getting released months before a game is released and we've had none for Agent since it was announced. They could be doing things differently with this game though.

Beyond Agent, Rockstar is currently promoting the Red Dead Redemption DLC. There's also L.A Noire from Rockstar on the horizon which is due out "Fiscal 2011" (1st Nov 2010 - 31st Oct 2011).

I think the first we heard of GTA IV's existence was at the Microsoft press conference at E3 in May 2006, however the first actual game info we got was the first trailer in late March 2007.